1. Field of the Invention
The present invention relates to a lightweight secondary battery with a high energy density.
2. Description of the Related Art
As note-type personal computers, portable telephone sets and portable terminals have been remarkably developed, lighter secondary batteries with a higher energy density are highly required.
A first prior art secondary battery is a lithium ion battery formed by a positive electrode active including lithium transition metal oxide such as LiCoO2 and LiMn2O4 as active material, a negative electrode including carbon as active material, and an electrolyte. In the first prior art secondary battery, a charging and discharging operation is carried out by absorbing lithium ions from one of the active materials into the other and expelling lithium ions vice versa.
In the above-described first prior art secondary battery, however, since use is made of heavyweight lithium transition metal oxide as active material of the positive electrode, the energy density is about 150 mAh/g, and is insufficient.
A second prior art secondary battery is a metal-sulfur type battery formed by a positive electrode including an organic compound having a sulfur-sulfur bond as active material (see: U.S. Pat. No. 4,833,048 and JP-A-74459). In the second prior art secondary battery, a charging and discharging operation is carried out by an electrolytic reduction to form a sulfur metallic ion bond or a sulfur-proton bond and an electrolytic oxidation to form a sulfur-sulfur bond. Since the positive electrode is mainly constructed by lightweight atoms such as sulfur and carbon, the energy density is about 330 mAh/g when the organic compound is 2,5-dimercapto-1,3,4-thiadiazole (DMcT), and therefore, is relatively high.
In the above-described second prior art secondary battery, however, the efficiency of recombination of the cleaved bonds is low, and also, the stability is low.
A third prior art secondary battery is a polymer battery having one or two electrodes including conductive polymers (see: U.S. Pat. No. 4,442,187). In the third prior art secondary battery, a charging and discharging operation is carried out by a doping reaction for doping electrolytic ions into the conductive polymer and an undoping reaction for undoping electrolytic ions from the conductive polymer. In more detail, in the doping reaction, exytons such as charged solitons and polarons generated through electrolytic oxidation and reduction are stabilized by paired ions. On the other hand, in the undoping reaction, the stabilized exytons are electrochemically reduced or oxidized. Since the positive and/or negative electrode is also mainly constructed by lightweight atoms such as carbon and nitrogen, the energy density is relatively high.
In the third prior art secondary battery, however, since the exytons generated through oxidation and reduction are non-localized in a broad π-electron conjugated region, so that the exytons interface with each other, there is a limit in the concentration of exytons, which invites a limit of the energy density.